Photo-luminescent composition consisting essentially of boric acid and fluorescein



.1. STEERUP 3,291,749

PHOTO-LUMINESCENT COMPOSITION CONSISTING ESSENTIALLY OF BORIC ACID ANDFLUORESCEIN Filed Aug. 14, 1963 INVENTOR.

f/7185 Seerup United States Patent 3,291,749 PHOTO-LUMINESCENTCOMPOSITION CGNSIST- ING ESSENTIALLY F BORIC ACID AND FLU- ORESCEINJames Steerup, Park Ridge, Ill., assignor to Triaxon, Inc., Glenview,Ill., a corporation of Illinois Filed Aug. 14, 1963, Ser. No. 302,204 7Claims. (Cl. 252-3013) This invention relates to a photo-luminescentcomposition and a method and apparatus of employing such a composition.

A luminescent material, as distinguished from an incandescent material,emits light at low temperatures after it has been previously excited bythe absorption of energy. While a luminescent material absorbs energyand subsequently emits part of this energy as light, a dye or pigmentwhen exposed to light energy merely reflects part of the light energyand absorbs the remainder.

The phenomenon of luminescence is believed to involve the absorption ofenergy that excites one or more electrons bonded to an atom or group ofatoms, at least part of the excitation energy subsequently being emittedas light. According to the quantum mechanics theory proposed for theluminescence phenomenon, an excited atom can exist indefinitely atabsolute zero. However, at higher temperatures, a given atom can beexcited to exist briefiy in one of several separate and distinct statesof higher energy, but not in any intermediate state. Each energystate ofthe atom corresponds to a diierent coniiguration of its electrons. Whenthe excited atom drops in energy from one of the sharply detined highenergy levels to a similarly sharp lower energy level, the difference inenergy is emitted as a discrete quantum of radiation or light energyknown as a photon. When isolated, all atoms are somewhat luminescent ifproperly excited. However, the energy level of an excited atom in amolecule is influenced by the one or more adjacent atoms which candissipate this excitation energy in the form of heat energy by atomiccollision or, where the excited atom is bonded to one or more of theadjacent atoms, by increasing the atomic motion or oscillations of theatoms relative to each other. Thus, atomic collisions which are requiredfor thermal light emission prevent luminescent light emission. Theprobability of dissipating potential luminescence emission energy, byconversion into heat energy, is enormously increased upon going from anisolated atom or molecule to a pure, elemental liquid or solid where theexcited atom is bonded to myriads of identical atoms. As would beexpected, therefore, most pure, elemental liquids and solids are eithernon-luminescent or are ineiiiciently luminescent.

There are many various types of luminescence, such as Chemiluminesceneewhich involves a chemical reaction,

usually oxidation;

Electroluminescence which involves excitation by electrons;

Radioluminescence which involves excitation by a radioactive substance;and

Photoluminescence which involves excitation by some form of light.

A photoluminescent substancev generally comprises a host compound and asmall proportion of an activator. The conventional host compounds areusually metal salts such as calcium sulfide, zinc sulfide, berylliumsuliide, cadmium sulfide and zinc silicate. Conventional activators areusually metals such as bismuth, copper, silver, manganese and chromium.Each activator is adapted for use with a specific host compound. As arule, the host compound without the activator does not exhibitluminescent properties. It has been postulated that the host l3,2%)1349Patented Dec. 13, 1966 ice compound functions as a medium in which tosuspend the activator atoms which, with the adjacent atoms ofV the hostcompound, provide discrete centres for localized absorption ofexcitation energy and subsequent emission of at least part of theabsorbed energy as light. If an excess amount of activator is employed,the centres become poisoned or incapacitated with the result that thesubstance becomes non-luminescent.

It is an object of this invention to provide a photo-luminescentcomposition characterized by high brightness and high degree of lightretention after the light source has been removed.

It is another object ofthis invention to provide' a photoluminescentcomposition which employs' a readily available, commercially inexpensivematerial as the host cornpound. Y

Another object of the invention is to provide a durable transparentcarrier for a photo-luminescent composition.

A further object of this invention is to provide a method and apparatusfor mobilizing a photo-luminescent composition so as to present theappearance of uid `ligh Another object of this invention is to provide aphotoluminescent composition in the form of a readily mobilized fluid.

Other objects and advantagesot this invention will become apparent uponconsideration of the following speciiication and claims when read inconjunction with the accompanying drawing in which FIGURE l is aschematic illustration of a suitable method and apparatus for mobilizinga photo-luminescent composition, and FIG- URE 2 is `an enlargedcross-sectional illustration of a preferred embodiment of the inventionin which the photo-luminescent composition is dispersed within a clearplastic coating.

The photo-luminescent composition of this invention employs boric acidcrystals as the host compound and fluorescein as the activator.Fluorescein has the empiri- -cal formula C20H12O5 and the structuralformula HO OH The activator, uorescein, may be employed in amountsranging from 0.001% to' 0.3% by weight of the host compound, boric acidcrystals. However, it is preferred to employ the activator in amountsranging from 0.02% to 0.1% by weight. l l

As the relative amount of fluorescein in the photoluminescentcomposition is increased within the stated range, the luminescent colorof the composition will change from blue to a blue-green of greaterbrilliance. Although it is preferred to use U.S.P. fiuorescein, anycommercially available grade of uorescein may be employed with somesacriiice in the brilliance of the luminescent effect.

In order to obtain an eicient photo-luminescent composition it has beenfound necessary to employ boric acid, as distinguished from salts ofboric acid. In addition, the boric acid must be in crystalline form.Although it is prefer-red to employ U.S.P. boric acid crystals,commercial grade boric acid crystals may also be used in the practice ofthis invention.

y Neither boric acid crystals nor iiuorescein by themselves possessphoto-luminescent properties. The same isl true of a mere physicalmixture of the boric acid crystals and uorescein. In order to obtain thephoto-luminescent composition of this invention, it is necessary to fusethe mixture of boric acid crystals and fluorescein, preferably atatemperature between 320 F. and 450 F. This fusion can be accomplished bymelting the boric acid crystals and thereafter adding the fluorescein or-by forming a mixture of the two components and thereafter melting themixture. As soon as the 'horic acid crystals are melted, the uoresceinrapidly and automatically disperses itself in the melted host compound.kAfter the mixture has been fused, it is cooled and, if desired, groundinto a powder of a particle size such that the crystalline structure ofthe -boric acid is retained.

If it is desired t'o employ the photo-luminescent composition in theform of a sheet, the fused mixture is cast as a sheet and slowly cooledto prevent -rapid crystallization which would cause cracks to appear inthe finished sheet. The cooled sheet has a glazed, glass-likeappearance.

The photo-luminescent composition of this invention is excited toluminescence by being exposed to light, both visible light andultraviolet light. Both sunlight and light from fluorescent lampsprovide excellent photo-luminescent elects. Exposure to infra-red raysdoes not cause the composition of this invention to exhibitphotoluminescent properties for the apparent reason that the -heat frominfra-red rays causes increased atomic collisions which as previouslymentioned destroys photo-lumi Descent emission. Thev photo-luminescentcomposition of this invention when exposed to a source of light willexhibit a high degree of brilliance and a luminescent retention periodof from live to ten seconds after the light source has been removed,i.e., under an extreme heavy input of light, it is possible to obtain aluminescent retention of approximately two minutes; The duration ofluminescent retention as well as the degree of Abrightness of thecomposition is directly proportional to the intensity of the lightsource and the distance between the light source and the composition,after excitation.

The luminescent properties of the composition of this invention areoptimum at room temperature. As the temperature increases, theluminescent properties of the composition decrease; however, a decreasein temperature does not have any adverse effect on the luminescentproperties of the composition. Even ifv the luminescent composition isexposed to temperatures high enough to render the compositionnon-luminescent, it completely regains its luminescent properties uponbeing cooled.

The photo-luminescent composition of this invention m-ay be produced asa fused mixture in the form of a cast sheet, in the form of crystalswhich may -be finely divided and/or compressed into solid forms, or as asuspension in a non-aqueous liquid. The composition may also be producedas a non-fused mixture permitting the manufacturer of a fabricatedproduct employing such luminescent composition to fuse the mixture whendesired. The mixture of boric acid crystals: and duorescein may alsocontain inert lillers, which are preferably transparent, vsuch as glassIbeads or glass fibers.

As boric acid crystals absorb moisture from the surrounding atmosphereand as absorbed moisture decreases the luminescent properties of thefused mixture of boric acidl crystals and uorescein, it is desirable toprotect the luminescent composition from the atmosphere or reactivatethe composition before use by heating to expel the absorbed moisture.,,The luminescent composition in the form of finely divided crystals canbe protected 'from the atmosphere by packaging thesame in hermeticallysealed containers or by dispersing the same in a non-aqueous liquidwhich is not a solvent for boric acid. It the photo-luminescentAcomposition is in the formof a sheet it can be protected `from Vthemoisture of the atmosphere by a clear protective layer of an non-solventImaterial such as acrylic resins,` paraflins, silicones, etc.,

Example I The following mixtures of -boric acid crystals and lluoresceinU.S.P. were formed and fused at 450 F. for approximately forty-liveminutes:

Sample Boric Acid Crystals, Fluorescein, Part Parts 1, ooo 2, ooo s, ooo4, uno 5, noo

After exposing the cooled samples to a light source, visual observationof the degree of the brilliance and duration of luminescent retentionindicated that all samples were good with Sample E being outstanding.

Example II v The following mixtures of boric acid crystals and uoresceinU.S.P. were prepared by the method of Example I:

duration of luminescent retention indicated that increasing theproportion of boric acid crystals -relative to lluorescein results in adecrease in brilliance and that the inclusion of glass bead lillers hadno adverse eiect on brilliance or luminescent retention.

The photo-luminescent composition can be employed in sheet form for thereproduction of -a light image. The fact that light entering a sheet ofthis composition does not diffuse is shown by the fact that light can bepassed through a half-tone negative and the image produced by thesubsequent lluminescent light emission is formed with extreme clarity.

In one especi-ally useful embodiment of the composition of thisinvention the boric acid-liuorescein mixture is incorporated into -aclear polyester plastic preferably of low viscosity. The fluidcomposition thus formed can then be molded, cast in sheet form, orsimply applied Yto .a surface in the form of a thin coating in much thesame manner as a paint. As the plastic hardens it forms an excellentprotection lfor the active b-oric acid-lluorescein mixture againstmoisture. Suitable clear polyester plastics lare available from GeneralElectric Company, Freeman .Chemical Company and Minnesota Mining andManufacturing Company.

Referring to FIGURE 2 of the drawing, the clear polyester plastic isshown as a relatively thin coating 10 having iine particles 11 of theboric acid-lluorescein -mixture uniformly dispersed therein. The coatingis shown applied to a base 12 which m-ay he a lboard or other supportingsurface, as a signboard.

The photo-luminescent composition may also be applied to a surface,which is preferably opaque or reflective, by either glazing thecrysta'ls on the surface or by dispersing the crystals -in alow-melting, transparent, non-solvent material such as parain a-ndapplying the dispersion to the surface. In the latte-r instance, theparaffin also functions as a protection a-gainst absorption of moisturefrom the atmosphere. In the former instance, a protective coating issubsequently applied. Although the thickness of the .coating may be ofmolecular dimensions, such as a one-ten thousandths inch film, excellentresults have been obtained with a coating having a thickness ofapproximately one-sixteenth of an inch.

A photo-luminescent composition can be mobilized by suspending thephoto-luminescent composition in a non-aqueous liquid, such `as mineralOil. In the case of a fused mixture of -bor-ic acid crystals andliuorescein, the liquid should be a non-solvent f-or the mixture.

The schematic ligure of the drawing illustrates a suitable method andapparatus for mobilizing a suspension of a photo-luminescent compositionin a liquid wherein the liquid suspension is contained in a reservoir 1.A transparent conduit 2 is connecte-d to the `reservoir 1 and suit` ablemeans such as a pump 3 is provided for passing the liquid suspensionthrough the conduit 2. A light lsource 4 is provided adjacent to thetransp-arent conduit 2. In operation, the liquid suspension is pumpedfrom the reservoir 1 through the transparent conduit 2 Where it isexposed to the light source 4. The light excites the photoluminescentcomposition suspended in the liquid so that the liquid suspension willemit luminescent light through the walls 'of the transparent conduitafte-r it has passed the light source. In this manner, conveyance oflight in a liquid form is achieved. It will ybe appreciated that thetransparent conduit can be made of a flexible material, permitting anumber of uses such as a flexible light probe for internal medicinepurposes, thereb-y avoiding the danger of glass breakage ,and heatinvolved in conventional light means employed for this purpose. Themobilized lig-ht of this invention can also be used for providingexplosion-proof lighting in locations such as mines and grain storagebuildings, where the dan-ger of gaseous explosion by electric dischargeis present.

Another application of the mobilized light olf this invention is inemploying a light source to impose an image upon the liquid suspensionand thereafter passing the excited photo-luminescent liquidsuspensionthrough the transparent conduit for viewing. Thephoto-luminescent composition of this invention permits such anapplication as it has -a relatively high degree of reproduction idelityof the image imposed by the light source.

In general, the photo-luminescent composition of this invention may beemployed wherever cold light emission of short duration is desired andwhere an excitation light source is available. Thus, the composition maybe employed as a coat-ing for a target used with a light pistol. Thecomposition may also be employed as an interior coating for uorescentlamps which conventionally are constructed in the form of an elongatedtubular envelope having electrodes sealed into the ends thereof andcontain -a gaseous atmosphere which may be a mixture of a rare gas andmercury. The interior coating is excited to luminescence yby ultravioletlight emitted by the discharge of the lamp. Application is also found in-safety devices Where road marks can be formed from thisphoto-luminescent composition by itself `or in a mixture with lareective ller. This composition may also be employed as a background orcoating for .a -ashing electric sign to ac-hieve unusual |lightingeffects and provide illumination in the intervals between flashes ofelectric light.

While the invention has been shown and described in the preferred form,it is to be understood that various changes and modifications may bemade therein by one skilled in the .art without departing from theprinciple of the invention and it is intended to cover all those changesand modifications which .come within the scope of the appended claims.

I claim as my invention:

1. A photo-luminescent composition consisting essentially of boric `acidcrystals `fused with Ifrom 0.001% to 0.3% fluorescein based on theweigh-t of said boric acid crystals.

2. A photo-luminescent composition consisting essentially of a hostcompound fused with .an activator, said host compound being boric acidcrystals and said activator being iiuorescein, the amount lof activatorranging from 0.02% to 0.1% by weight of the host compound.

3. A composition which becomes photo-luminescent when fused consistingessentially of boric acid crystals and from 0.001% to 0.3% liuoresceinbased on the weight of said Iboric acid crystals.

4. A composition which becomes photo-luminescent when fused `consistingessentially of a 'host compound and an activator, said host `compoundbeing boric acid crystals and said activator being iluorescein, theamount of activator ranging from 0.02% to 0.1% by weight of the hostcompound.

5. A method of making a photo-luminescent composition 4consistingessentially of the steps of fusing a mixture of boric acid crystals andlluore'scein and cooling the fused mixture, the amount of iiuoresceinran-ging from 0.001% to 0.3 by weight of boric acid crystals.

6. A method of making a photo-luminescent composi tion as dened in claim5 wherein the amount of uorescein ranges from 0.02% to 0.1% by Weight ofthe boric acid crystals.

7. A liuid photo-luminescent composition consisting essentially of afused mixture of boric acid crystals with from 0.02% to 0.1% iluorescein'based on the weight of said boric acid crystals, suspended in anon-aqueous liquid carrier which is a non-solvent for the fused mixture.

References Cited by the Examiner UNITED STATES PATENTS 2,298,968 10/1942Roberts 252-301.3 X 2,519,825 8/1950 De Ment Z50-71 2,936,371 5/1960White et al. 250-71 2,945,128 7/1960 Sun et al 250--71 X RALPH G.NILSON, Primary Examiner.

ARCI-IIE R. BORCHELT, Examiner.

2. A PHOTO-LUMINESCENT COMPOSITION CONSISTING ESSEN-TIALLY OF A HOSTCOMPOUND FUSED WITH AN ACTIVATOR, SAID HOST COMPOUND BEING BORIC ACIDCRYSTALS AND SAID ACTIVATOR BEING FLUORESCEIN, THE AMOUNT OF ACTIVATORRANGING FROM 0.02% TO 0.1% BY WEIGHT OF THE HOST COMPOUND.